Measuring apparatus

ABSTRACT

Apparatus for providing an absolute indication (that is, an indication which is not lost due to interruption of the supply source) of the position of a movable object, such as a machine tool, by means of a series of binary digital elements extending throughout the range of movement to form a chain-code sequence of dissimilar digital patterns. Each position of the tool is identified in dependence on the particular one of those patterns which is then in registry with a reading head, the identification being in terms of the number of electrical pulses needed to actuate a chain-code generator to produce a similar pattern.

United States-Patent Kennedy et ai.

1541 MEASURING APPARATUS [72] Inventors: Christopher John Kennedy; Alexander Turnbuli Shepherd; Graham Isaac Thomas, all of Edinburgh;

Scotland [73] Assigneez' Ferranti, Limited, i-iollinw'ood, Lancashire, England [22] Filed: Jan. 5, 197 1 211. Appl N0; 104,095

[30] Foreign Application Priority Data Jan. 8, 1970 Great Britain ..'...902/70 5 21 u.s.c1. .340/347 inns/151.11 [51] no. or. ..G08c 9/00 58 Field of Search. ..340/347;23s/151.11-

[56] References Cited IE SIAIES.PAIENT 3,461,449 8/1969 Baskinetai. ..i... 3 40/347 51 Nov. 7, 1972 3,531,793 9/1970 bureau ..340/347 1 Primary Examiner--Maynard R. Wilbur Assistant Examiner-Charles D. Miller Attorney-Cameron, Kerkam & Sutton [57] i ABSTRACT Apparatus for providing'an absolute indication (that is; an indication which is not lost due to interruption of the supply source) of the position of a movable object, such as a machine tool, by means of a series of binary digital elements extending throughout the range of movement to form a chain-code sequence of dissimiiar'digital patterns. Each position of the tool is I identified in dependence on the particular one of those patterns which is then in registry with a reading head, the identification being in terms of the'number of electrical pulses needed to actuate a chain-code generator to produce a similar pattern.

LOCK CONTPGL ams/rte flaunts/ PATENTEDnnv'mn- I 3.702.471

snmunra j 345745.42 Reset Set I #2 //3B i II A 05 responsive to the position of a" .tion of the system,

tio'nal indication after apparatus responsive to -the position of a relative position of the first .1 MEASURING-APPARATUS This invention relates to measuring apparatus first object with respect toa second object;

. The invention'haspa'rticular re'lferenceto the numerical measurement of machine tool'movemjent where the first object is the framework ofthe' machine,-acting=-as-a 1 fixed reference structure and the second object is a movable part, such as theworktable. The invention will accordingly be described. :in that connection; but it should "be -;understoo'd restricted to such uses.

that the invention is rnot ltis known toobtain such pos itional information'by means ofapulse countwhich indicates the extent of movement from some reference point-Known arrangements of that kindyunless' involving an inconveniently large number'o'f tracks which extend thefu'll length of the movement, do not usuallyprovide-an absolutejreadv interruption of the energizaing, in the' sense that any such as thatdue to-a noise transient or ,a short failure of 'the-mains', causes the count to be lost'withno memoryin existence to restore the posithe interrupti n.

An object of the invention'isto providemeasuring apparatus capable of supplying in absolute formf the sortofpositional information above referredto with the use of only two full length tracks. U

' ln accordance with the present invention," measuring first it object movable relative toa second objectinclu'des 'a code member secured to the first object and carrying a sequenceof binary digital elements-whichextends in the direction of relative movementand is such that any group of a predetermined numberN of consecutive elements differs in pattemfrom any other group of Moonsecutive elements, and identifying ineans.

A staticizor stage, transfer meansfor transfe'rrin'g'to the staticizor stage at any given moment the digitalpattern of the particularsgroup which; representsthe'said objec tlz-at that moment, a code generator operable 'onipulse energi'zation toset up successive ones of said group pattems, a source for pulse-energizing thecode generator, comparison means interconnected with the sta'ticizorstage and the'code generator so as to respond *when theirpatterns'are the same, and output means responsive tosa'idsotirce and the comparison means to] derive a measurement signal which represents the rela'tive position of the first object by the number of pulses required to step the generator a machine'tool relative to provide an absolute indication of that-position includes a codemember secured to thefrarnework adjacent to first scanning "meansin the form of apart-turn helical slot 12 (seethe illuminated slotl 2)to'-scan drum' 213A. It may conveniently coaxial with thel8 of the drum." In FIG. 1 it is con- "cealedby' bearing brokencircle 19., 20

-"nary digital elements in the formfof parallel straight linesfl he' sequence extends in thedirection of movemem, to'which the lines ar represented" by the presence 2 predetermined number N of in pattern from any othersuch group.

the fixed framework so as to nthe formof'an opticalgratin'g 11 FIG. 2) inthe otherwise opaque curved surface of a "drum 13AfThe drum is journalled in bearings 14 (FIG. 3)"car"'ried"'by supports 15 from-the worktable 16 to be driven bya motor l7,"also' secured to the table, about an axis aligned.

directionis normal to the plane of the paper in FIG. 1

but parallel to it" in FIGS. "2 and '3. In FIG. '2, the

bearings of the drum are omitted for clarity.

- Slot l2 is irradiated by a light sourcewhich is within be of tubular form 14 butits position is indicated by the "Gratingll is ruled" so -'as to carrya sequence of binormal, and'each digit is or "absence of a line. The

such that'any group of a consecutive digits differs sequence'forms a chain code Chain codes and the methods of generating them'in electronic form are described in a papertitled Chain Codes'and their Electronic Applications by'Heath and Gribble in VolflO8, Patt C, 1-96l,of the'Proceedings of the Institution of Electrical Engineers.

in the present example N is 14. The patterns are only approxirnatelyindicated in FIG. 2. The length of each group or pattern of l4"digitsin the direction of movement is such" that when' a" y *with drum 13A,*a part revolutionof the drum causes itspattern, digit by digit 'in' turn, from'end to'end. I v I A'photo'electric device in the form of a photocell '21 (FIG.;1) is carried-by the table '16 and disposed so as to {be irradiated by the light from grating l 1 when scanned from a datum pattern until the comparison means responds as aforesaid.

In the accompanying drawings,

FIG. 1 is a simplified schematic diagram'of one em-' bodiment of the invention, g

FIGS. 2 and 3 are views taken in the directions ll and III of part of the apparatus of FIG.- 'l,; 1,

FIGS. 4, 6, 7, 8, and 10 are diagrams'of furtherembodiments,

and 1 FIG. 9 shows an alternative arrangement of a part of a no. 5 shows a detail of the e llient of we. 4,.

form by way of example, see FIGSQJ ma, measuring apparatus responsive to the position of the worktable of preferred embodiment by helical'slot 12 and therefore be responsive to the complete pattern of l4di gits as so scanned. in FIG. 2 the position of the-photocell 21 is indicated by a half circle In-operation, therefore, any given relative positionof the table and the frame will be uniquely represented within'theresolution of the patterns by the particular "pattemwhich is then in register with drum 13A and hence is;scanned by the illuminated slot '12 at each revolution of the drum. As a result, the relative position of table and frame is indicated by a series digital signal derived by the photocell, this signal being repeated each time the drum rotates.

In the present embodiment of the invention, that signal 'is caused to provide a comparatively coarse measurement of the positiom'ove'r the very wide extent allowed by a l4-digit pattern code. Before the utilization means for deriving this measurement isdescribed, however, further equipment in accordance with the of the invention for deriving a I a comparatively narcornparatively fine readingover row range will be briefly indicate A' control grating 22 ruled with straight lines at equidistant spacings and substantially normal to the in the direction "of movement. This particular-'fgroup is in'register direction of movement is fixed to the frame .10 in re- Conveniently, drum 13Bis a continuation of drum 13A and so shares its bearings 14 and the driving motor 17. Illumination of the grating is provided by a light source which is not shown but which is located outside the drum and reaches the grating byreflection fromthe trace 23. r

A control photocell 24 is attached to the table 16 so as to respond to the grating 22 as so irradiated. In FIG. 2 its position is indicated by a circle.

To provide for reference purposes a similar pattern to that supplied by the control grating, a reference grating 25 (FIGS.- 1 and 3) ruled similarly to the control grating 22 is secured to the worktable so as to be fixed relative to the drumassembly. It too is illuminated by reflection from spiraltrace 23, using the-same. or a different light source (not shown) to that which irradiates the control grating. Areference photocell 26 is attached to the worktable-to respond to grating 25.

Hence in operation, as the drumrotates, gratings 22 supplying pulses at a nominal -l MHzfrequency. A fourth signal is. derived from the start signal from photocell 34.:- v

willalso be described later, the function of the 5 gating system 42 is to control the passage of clock pul- ,ses-'from.clock 43 to a three-decade counter 44 in de- -.pendence-upon the relating phase between the control and reference signals from cells 24 and 26.

After division to l KHz in a three sta g'e counter 45,

the clock train is applied together with the reference signal from 'cell 26 to controlthe clock frequency by means of a phase and frequency'locked loop compris ing stages 43 and 45 and afurther stage 46, so that one cycle of the reference signal coincides with precisely l,000 clock pulses.

The clock pulses are also appli d by way of an inhibit gate 47 to step a 14-bit chain code generator 51 and a four-decade counter 52. The pattern held in stage 51 and, that held in stage 41 are applied to a comparator stage 53 arranged to supply a'signal to close gate 47 when the respective patterns are the same.

Staticizor 41, code generator 51, pulse source 43,

' comparator 53 and counter 52, together with transfer and set up cyclic control and reference patterns of 25 means in the form of the arrangements for scanning the relative phase dependent on the relative position of the objects over the length of one, cycle; and the corresponding photocells 24 and 26 develop signals of that relative phase.

To define repetitive periods of scan for both the code and controlgratings, a drum 13C, conveniently'combined with drums 13A and 13B, is provided. Its curved surface carries a slot 31 (FIG. 3) parallel to axis 18 and illuminated from within the drums as is the spiral slot 12. To cooperate with this is a mask '32 attached to the worktable l6 and having a slot 33 similar to'slot 31. Outside the maskvand also attached to the table is a photocell 34 omitted from FIG. 3 so as'not to confuse the drawing. The arrangement is such that the cell is irangular position of the composite drum 13A/C is such that slot 12 on drum 13A is aboutto start scanning the particular chain code pattern then in register with that drum.The signal from cell 34 may therefore be used to effect the required definition of the scan periods; it will be referred to as the start signal.

Suitable arrangements responsive to the code and control signals so as to indicate by a coarse and a fine display the relative position of the table will now be described with further reference to FIG. 1.

The signal derived by the code photocell 21 from the chain code grating 11 as scanned by slot 12 is applied to a 14-bit staticizor stage 41 arranged to store the series signal received from the cell in response to a scan of the pattern then in register with drum 13A. The stage is in the form of a shift register to the Input point of which the signal is applied.

As described later, each .scan is initiated by a start signal pulse from cell 34, indicating that slot 33 in mask 32 has reached alignment with slot 31 in drum 130.

The signals from the control and reference cells 24*" to the gating system isderived from a clock stage 43 code patterns and applying them by way'of photocell counter 52 will have a range from l to 999 mm and counter 44 a range from 0.999 to 0.00l mm. These are the coarse and fine ranges above referred to.

' The code grating 11 and the control gratings 22, as already explained, are both fixedto the tool frame. The respective rulings are so aligned that the corresponding signals are in synchronism, that is to say, so'that each cycle of the control signal coincides both in space and time with a digit signal from the code pattern.

, synchronism is made useof to ensure that a radiated by the alignment of slots3l and 33 where the 45 are derived fromfthe positive-going crossovers of the squared control signal from photocell 24 and applied to the shift input of stage 41 by way of a strobe gate 54 under the control of the reference and start signals. As

will be appreciated later, it is gate 54 and its controlsin particular that provided by the start signal-which act as the group-selecting arrangements to apply to the staticizor the required pattern group included in the portion of the code member scanned by the drum.

The unambiguous changeover of the coarse digit 55 scale as the fine digit scale changes from 0.999 to 0.000

is attained by ensuring that the start of each scan of the chain code is determined by the first control signal strobe pulse which follows the first positive-going crossover of the-reference signal subsequent to the start signal from photocell 34.

This sequence of events at the start of each scan period is regulated by the strobe gate 54.,lt controls by way of the Shift input to the staticizor-the entry of the binary digits from the code-pattern in responseto the strobe pulses derived from the control signal, and to the scanwstart pulsesderived from the start and reference signals from cells 34 and 26.

*that the worktable isat the datum I The start signal also serves to reset and the counter 44 and 52. In describing the operation of this equipment, it will stages 41- and5l be assumed for convenience that'the pattern length N is only four digits, not fourteen. As the number 0000 cannot be used in a chain code, the datum or -position of the workpiece is taken to be the pattern 0001. v will be referredto as the zero position or datum pattern. The firstfew positions, shown as distances in from datum, are represented by. patterns as follows:

par-ran Pattem 0 v a 0001 l 0011" 2 0111 3 a "-1111 4 1110 5 1101 The last position, 14mm from datum, -is represented the pattern 1,000.

The action of the chain-code gof theapparatus will be described first.

. that decimal number ceris onding to the four-digit binary code at the new code pattern that has been v scanned and read into staticiz'or '41.

The first start "signal {generated is: then zautomatically employed to'resetcountersand regfisterstolzeroand initiatethereading of.thetcode=- pattern. r

T he first imam o"f drum i'l3A after the ifirstrstart signal scans the code grating landbdelivers to" the staticizor 4.1-1the particular :pattern 000 1 asselectediby the start signal'by wayof gate 54. As this pattern is same as that in stage i5 l, comparatori53closes= gate 4.7

.in timeito prevent a pu'lsefrom thefclock 'fromreaching counter 52,which thereforeremains indicating 000? The next start signal is employedto reset stages 41 andf'5l, restore the counters lto zero, and initiate :the nextscanperiod. I I f So long as-the:table remains in ithe position car- Assume, further, that when the table has reached,

say, a position 6 mm from datum, represented by the pattern 1010, with counter 52 accordinglyholding the number-f6" at the end of each scan periodrsome incident,such as a mains failure, causes the wholeequipment tobe de-energized. t Y Its state is then as described above for the off condition,'except that the pattern alignedwith drum 13A is l010instea'dof000l. 7 Y

Though this reversion will have emptied counter 52, thepositionindicationis not lost but is remembered" by the iabove-mentioned alignment of pattem I010 withdrum 13A. Hence assoon'as the power is restored, the first start signal resets the countersand stages 41 and 51 and the first rotation of drum 13A re-scans pattern 1010 of grating 11 and so returns it to stage 41. As stage 51 has been reset ;to the datum pattern 0001, comparator 53 opens gate 47 and keeps openuntil sufficient clock pulses have-stepped stage 51 to pattern 1010. The six pul- -.ses needed to. do this have also passed to the counter 52, thereby restoringthe indication 6.

absolute indication of thetool position has thus :been provided. I

The operation is closely similar where the pattern is of l4-digit length, allowing an indication of the tool position form datum to be extended to nearly 10,000 mm'thelimit prescribed by counter 52. vj -'I*hereading thus arrived at is accurate to the pitch of the. code grating. Thefine-reading part of the equip ment operatesfin reliance on the .fact that the space between adjacent rulings of the control grating and hence of the code grating is spanned byone cycle of thecontrol signal; the phase .difference'between that signal responding to thepattem'OOOl, 'repeatedscansof the code pattern by. drum 13A after eachresetleaves stage 53 holding :that pattern, with counter :52 indicating 000"at theend of each scan. period.

Suppose, now, that, {the table "moves forward by 1 1 mm, thereby changing the pattern" to 0.011 Aftereah start signalhas resetall stagesythe first revolution of the drum scans thisxnewpattern anddelivers it to the staticizor 41. As the code. generatorSl been reset to the datum-pattern 0001,thecontents of. stages'41 and, 51 are now dissimilar; accordingly thecomparator 53 opens gate 47 to' allow a clockpulse tqpasslfrom tlie source '43 to .the generator. Thisliastwoeffectsi first pulse to pass causes generator- 151tobesteppedto the pattern 0011', thereby causinggate 47 to'close; :7

and counter 52 isstepped to the=nuinber l As the table continuesto:movefithis.processiscontinued; at each new position, the first-start signalfrom .cell '34 resets the counterrsland stages41and 51sDuring the ensuing scan period thecounter is steppedcto and-the reference signal is measurable to a thousandth of thatdistahce as follows later.

As all'therelevant stag'esare reset before each active I scan, the apparatus responds with equal effect should the direction of movement be reversed.

To allow for the sudden .jump from one code pattern =-to the next as the table moves, the slot :12 isv designed to :scanone more digit of the chaincode-that is,'( N+l) for11'5 digits, rather than the 14 ofthe actual pattern. I

lfi'in additiontit should berequired that the readout from'thecode scale should be maintained during the movement ,of the scanning drum, the physical length of .the l4 digittco'de pattern which must be scanned would appeal to stretch or contract, depending on the direction ofmovement of theworIrtabIeEI-Ience the extenti'of slot 12 inthe axial direction must besufiicien't :toscan the 1.4.or15 digits at the fastest required rate of that movement.

'TThus, for'example, if the-apparent velocitycomponent of theslot 12 along the. length of the codescale when thetable'is at ,rest' is V, and the maximum required :velocity 0.5V, then the effective scanning velocity of the'slot willvarybetween V0:5V and -V+O;5V-'tha't is tosayfthe "maximum required extent of slot '12will be 'llSjti'rnes' thatrequiredxwhenthe ttableisatresti Where the slot extends ar'cuately for more than a sin- I vgleturn, it is necessary to use only alternate ones of thebe followed by another before the readout is finished.

By. using only alternate start signals, these second o'nes are suppressed. l

v A further vadvantage of an (N+l) scan isthatthe provision of an (N+l )th stage in the staticizor and the use for the zero signal of the code pattern digit 1 followed byN digits allows the filled condition of the staticizor to be checked; for in such an arrangement the arrival of the digit 1 in that extra stage (as'indicated in any convenient manner) shows that the staticizor contained a complete code pattern. It would not be necesf sary to add an extra stage to the code generator 51: it would still posses only N stages, which the comparator I 53 would compare with the original first N'stages of the staticizor. 1 1 A possible additional advantage of an (N+l) scan could be the availability of the (N-H)th stage to provide a general reset signal, instead of using the start signal for that purpose. Gate 42 is designed so as to pass the clock pulses through to counter 44 throughout each period defined by the positive-going crossover of the reference signal and the ensuing crossover 'in that direction of the control signal. The phase difference between them is thus digitized. As already mentioned, the clock frequencyis so controlled by the phase and frequency locked loop 43, 45, and 46 that each cycle of the reference signal carries exactly a thousand pulses. If then, for example, the signals are inquadrature, representing a distance equal to a quarter of the grating pitch or 0.25 mm, gate 42 is open long enough to pass 250 pulses to the counter ,and so produce the correct reading 9.250 mm. This reading is also of an absolute nature, since on the occurrence of a break in the energization,.themeasurement within a grating pitch will be remembered by the relative positionsof gratings '22 and 25; so the first rotation of drum 138 when the ener'gization isresumed will restore the two signals and so allow sufficient clock pulses to pass to the counter to digitize their relative phase. l

It is true that this digitized reading will be ambiguous inasmuch that the particular rulings are not identified, but such identification is provided by the comparatively coarse reading of counter 52. v

Hence, in operation, the position of the worktable along the considerable length of the code grating is defined to the nearest ruling of that grating by counter 52, while the position between rulings is defined by counter 44-both readings being absolute.

' Using 14-bit patterns with gratings of 1 mm pitch, the maximum length of travel, expressed in mm, would be 2': less the unusable all-zero pattern and less the axial length of drum 138. This works out at abut l6'meters.

As within the. whole of that long range an absolute mea-- I surement to a thousandth of a mm is readily obtainable as described-above (provided that counter 52 has suffi- 'cient additional stages of higher significance) the advantage of a measurement. system in accordance with this embodiment of the invention is clearly shown.

and 51 and counter 52.

,j, Stages 42 and 54 may consist of simple arrangements of bistables and gates. Various circuits may readily be pdevised to" suit the equipment available. The code s gen'erjato'r.5 1 may take the form of 'a shift register com- ...--bined-=.'\vith=csuitable logic togenerate the required N patternsiS/arious circuits for this purpose are disclosed ,in the I-Ieath and Gribble paper above referred to.

Various features of the above-described embodiment may be modified within the scope of the invention. Thus the scanning system may be modified considerably-such as by the substitution for the mechanical device of a rotating drum of a static system in which the scan is effected by lamps'sequentiallyenergized in polyphase or a single modulated lamp with multiple cells selected in polyphase. I

*The light source which irradiates slot 12 would preferably be outside the drum, the slot 12 being replaced by a helix of reflecting material, as are helices Instead of being normaltothe direction of movement, the code lines of grating 11 may slope a little, to match the slope of helix 12. n I 1 The eflective crossovers of the control and-reference 25 signals'may be the negative-going crossovers, rather than the positive-going. Where the fine reading is not required, the circuit of FIG. 1 may be simplified as-shown'in FIG. 4, where the components already referred to are indicated by their previous references.

The drum 13A operates as before in association with the code grating 11 to serialize the code patterns and deliver them to the input point of staticizor 41, this time by way of a two-entry AND-gate 61. The drum itself is not shown.

0f the fine-reading equipment of FIG. 1, control and reference gratings'22 and 25, photocells 24 and 26, and the scanning helical trace 23 of drum 13B are omitted.

pulses. .These strobe pulses are applied by way of another two-entry AND-gate 64 to the shift input of the staticizor4l. I

The other two inputs to gates 61 and 64 are supplied by the Set output of a bistable 65. The stage is switched to that condition by each start signal from cell 34 and reset by a signal from an (N+1 )th stage 66 added to the staticizor.

The pulse generator or clock 43 is supplied as before; but this time it is free-running rather than being synchronized to a grating pattern. Its pulses are applied by way of an AND-gate 67 to the inhibit gate 47 and thence to generator 51 andcounter 52. The other input to gate v67 is supplied by stage when in its reset state. The start signal is applied, as before to reset stages 41 'Iheeperation needonly be briefly indicated.

Each start signahinaddition to its various resetting duties, sets bistable 65 thereby alerting gates 61 and 64' to pass the serialized code signals and the strobe pulses through to the staticizor. As indicated to a verymuch exaggerated scale in FIG. 5, the markings'of the strobe train are located so that the train, acting by way of the shift input. to the. staticizor, selects just thecenter-of each codepulse (if any).

jWith stage 65 in its Set condition, the pulses from clock 43 are blocked'at'gate 67 1 r 2 With the" arrival of digit 1' at stage (N+1). 66. hereb signalling that the staticizor. contains the complete pat tion-whenthe'cell to which it is connected'is reading digit l of the code pattern; when the digit that is read is tern of the code group scanned,'bistable 65 is .reset,

be sufficient to allowthe counter to keep pace with the fastest likely movement of the table, but is otherwise not critical.- z 7 Where it. is not convenient to'provide arotary for other form of repetitivescannenthe transfer means maybe modified to transfer the N digits of agroup to 'the staticizor in parallel rather than as a series train.

Such an arrangement is shown in FIG-6.

The code'optical grating 11, assumed as before to be fixed to the worktable and carry four-digit patterns, is irradiated by a stationary lamp 71,=-thelightfrom which is shielded to confine it roughly to the four elements of the particularpatternwhich represents the position of the table at the particular moment illustrated.

Irradiated by those four elements is a set 72*of four photocells, one for each digit of "the pattern. As with FIG. 5, the pattern is depicted to aimuch exaggerated scale. In practice, wherethe jpattem elements are spaced more closely than is practicable for the cells, the cells may be spaced as convenient and me light conveyed to them-from the repetitive code elementsiby some optical means, such as optical fibers; these are not shown. g a In order to avoid errors due to reading 'the'cellswlien they are half-way between adjacent digital'elernents 0, the'stage is'iriits Reset state. I

Comparator 53, code generator 51, and counter 52 may be as before, and thelpulses for operating the generator and the counter are againderived from a clock 43, connected'to stages 51 and counter 52 by way of gate 47. In the absence of a moving scanner, the

reset signals are derived from the clock by way of a stage '81 designed to pass} every nth pulse only; these are-applied to reset the generator and the counter. Each is also applied to the staticizor, where it causes the stages 77 to read the-pattem represented by whichever set of photocells .is switched through by stage 76. d d

The pulses subsequent to each nth pulse drive the code generator 51 and counter 52 much as'in the previously described embodiments until the pattern in the generator matches that in the staticizor; whereupon. the comparator blocks gate 47. e I v To prevent the nth pulses reaching the generator and the counter, a. further inhibit 'gate 82 is inserted rather than aligned centrally on them, a second set'73v of four cells is provided, spaced by' halfa digit width to the side of set 72. In thedrawing, the cellsof set 7 2 are centered onthe digits. whereas those of set 73 are halfway between adjacent digits.

To select the .set which at any given moment isthe more centrally aligned-eg, set 72 as depicted-a second optical grating track 74 fixedto the worktable is provided. It is similar to track 62 of FIG. 5 and irradiated by lamp 71, or by a separate "lamp if more convenient. j a

Track 74-is read by a photocell 75 the signal from which is applied to control a switching stage 76 by means of which the sets are connected to a staticizor 41 Cell 75 so controls stage 76 asto select forapplication to the .staticizor at any given lrnoment the particular one of sets which is the more centrally aligned on the code elements at that moment-set;72 in this example. The staticizor is given the reference 41 to distinguish it from staticizor dl of theabove described embodiments, as the two are not quite alike in structure. Staticizor 41 consists of N (four) bistable stages 77 to which the four cells of the selected set are connected.

Each stage is arranged to switched to its Setcoridia between the clock and gate 47 and arranged to be closed-during eachnth pulse. t v

As in the arrangement of FIG. 4, the clock is free running. The frequency and the value of 1: should be such as to-allow-all the four possible codegroups to be set up in turn in generator 51 before the fastest likely movement-of the worktable has brought the next code group to the read position-in other words, in the pre'sentexample, before the table has moved sufficientlyfor the cells of set 73 to be more centralized than those of set 72 andso ready to be switched to the staticizor. 7

Further description of the operation is hardly necessary. As'the table moves steadily'in one direction, the photocell sets 72, and 73 are alternately switched through to the staticizor. During the brief interval throughout which ,a set is so switched, an nth pulse from the clockcausesthe stages 77 to take up theconditions representing the digits seen by those cells. The ensuing pulses bring stage 51 to duplicate the pattern in the staticizorand enter the appropriate number of pulses into the-counter. The clock runs quickly enough to have finishedthis process before the other set of cells is brought to the read position. v e

. As the counter and generator are reset before each pattern is read, the apparatus responds equally should 1 the direction of movement be reversed.

Where, it is desired to supplement the reading derived from the code track, the arrangement of FIG. 6

may be modified as shown in FIG. 7.

Here the grating 74 of FIG. 6 is replaced by a grating 83 which, like grating 11, moves with the workpiece and'which produces in combination with a short fixed index grating 84 a cyclic pattern 85 of wavelength grating .11 andillumined by the form of a ring potentiometer, the photocell signals are converted into pulse form by a stage 91 and applied over Add/Subtract leads A and S to a counter 44, corresponding to counter 44 of FIG. 1. i

As described in the. above-mentioned specification the arrangement is suchthat for each cycle. of pattern movement, pulses are applied to the counter, to be added or subtracted in accordance with the direction of movement. Hence thereading provided by counter 44 sub-divides by 10 the reading provided by counter 52.

As the signal derived in stage 87 is responsive to the movement of the code track 11 as well as to the movement of track 83, it canbe used to provide what timing control in dependence on the worktable movement is required by the code identifying part of the equipment.

Thus track 74 and photocell 75 may be dispensed with and the signal for controlling the switching stage 76 derived instead from the appropriate phase point in stage 87. I

The remainder of the equipment of FIG. 7, concerned with the measurements derived from the code scale, may be asin FIG. 6 and is therefore not shown. If a more accurate measurement to supplement that from a code rneasurement of the parallel read-out kind, as described with reference to FIGS. 6 and 7, is required, it may be necessary to use a dynamic system. This may be as shown in FIG. 8. v Control, reference, and start signals are derived by photocells 24, 26, and 34 as in the arrangement of FIG.

l, the drum and associated gratings being omitted to simplify the drawing. From those signals, the pulses for counter 44 may also be derived as in the FIG. 1 ar rangement. Here again, these more accurate signals may be used to control the timing of the code identifying apparatus. In this example, the clock 43, -.which is free-running in the embodiment of FIG. 6, is controlled by the reference signal as in the embodiment of FIG. 1.

Similarly the control of switching stage 76 to select the appropriate set of photocells is exercised by a signal derived from the 0.5 digit stage of counter 44 and which consequently occurs twice in each code group.

. Again, the signal which transfers to the staticizor 41 the group reading derived by the active set of I photocells is supplied by the first reference pulse which follows a start pulse. a

' To derive this transfer signal, each start pulse is applied to set a bistable 92, thereby alerting an And gate 93 to pass to the staticizor the next reference pulse,the staticizor is reset by the next control pulse.

Other resetting operations may be derived, as before, by the start signal.

It is not necessary for the staticizor and code generator to be separate stages; they may be combined in the one unit, and the measurement derived from the number of pulses required to step the generator from the staticized pattern to the datum pattern, instead of in the other direction as in the embodiments so far described.

The principle of operation is that, during each readout (which for the moment is assumed to be of the serial kind), the unit is arranged to act as a shift register, operating from a datum pattern such as 0001. Ori completion of readout (which may be identified by the entryof digit 1 into an (N+1)th stage as above described) the unit is converted by a switching operation into a code generator, while retaining the staticized code pattern. This generator pulsed back to the datum. patternby the same sort of clock pulses as actuated generator 51, these pulses being also passed to a couhterb'lhe return of the unit to the datum pattern is detected by the comparison means; this in effect compares theactual state of the unit with the datum state and arreststhe'flow of pulses when they are equal.

The number of pulses needed to effect this restoration represents the code group that is scanned.

All that the changeover does is to reconnect the bistable stages of the register to one another, adding appropriate feedback, to form a code generator appropriate to the value of N.

An arrangement switched to the particular embodiment of FIG. 4 is shown in FIG. 9.

The combined unit, with its (N+l )th stage, is indicated at 101. Associatedwith it is a logic network 104, designed to effect the necessary switching.

The code pulses are applied, to what is the input of the-unit when connected as a shift register, byway of an AND-gate l02arranged to be alerted by a bistable 103 transparent'disc 113 rotated by a motor 117 about an when in its reset state. In its set state, stage 103 alerts gate 67 between the clock 43 and inhibit gate 47, from which the pulses pass to the counter 52 and to the shift input to unit 101. The signal to set stage 103 is derived from the (NH )th stage of the unit. a

The function of theunit is determined by logic network 104 actuated by a function signal from bistable 103 so as to connectthe unit as a shift register when stage 103 is in .its reset state but as a code generator when stage 103 is in its set state.

The actual condition of the unit is compared by some comparison means 105 (which may include a simple diode network) with the datum condition, and'which acts to block gate 47 when the datum condition is reached.

Stage 103 is reset by the start pulses.

In operation, at the start of a scan,stage 103 is in its reset state, therebyacting through logic network 104 to connect the unit to function as a shift register and alerting gate 102 to allow the code pulses to pass into the register in synchronism with the strobe pulses from gate 54 on the shift input. During this process, stage 103 maintains gate 67 closed, to block the clock pulses.

When the entry of the code signal is complete, the resulting actuation of the (N+l )th stage switches stage 103 to its set state. Acting by way of the logic network, stage 103 converts the unit to its code-generating function, and at the same time allows the clock pulses to pass through gate 67 to actuate the code generator by way of its shift input. The return to datum is detected by stage 105, which arrests the flow .of pulses at gate 47.

In systems employing a parallel code readout, such as shown in FIG. 6,'the arrangement may be much the same in principle. About the only difference of importance is that, in the absence ofa (N+l )th stage, the function determining signal has to be derived in another manner-perhaps by using the trailing edge of eachiNth pulse, after it has caused the photocell reading to be transferred to the staticizor.

As shown in FIG. 10, the rotary scanning drum 13 of the arrangement of FIG. 1 may be replaced by a partly axis 118 and carrying circular tracks 113A. 111313, and

In this description, components corresponding to responding to theworktable l6 ofFlG, 1)or other object the movement of which is to be measured.

Disc 116 carries circular tracks flll'and 122. Coaxially outside it are annular tracks i125 and l34, which arefixed.v v V v .J

Thediscs are illuminated by a lamp 119 the beam fromwhich is collimated by sleds-94 to be projected through the discs and annuli to abank of .photocells 121, I24, 126, and 134.

' The track 113A is opaqueexcept for asingle transparent radial slot ll2'(corresponding to the'helical slot 12 of FIG. 1 which as disc 113 rotates, scans the chain code in the formof a radial grating on track 1 1 l.

Track; 1138 carries a continuous-uniformlyspaced radial grating 123 to scana moving controlgrating on track 122 and a fixed. reference grating on track 125.

Track 1 13C is opaque except fora single transparent slot 131 to generate'a start signalwhenaligned'witha transparent slot 133 in the otherwise opaque .traclc 132. The operation of this scanner will be readily ,apparent owing to the close analogy with the scanner of FIG. 1. V 1

vVarious features of the above-described embodiments may be varied within the scope of the invention. For example, the scanning means need not necessarily be a rotating drum or disc, but could be derived from a cathodesray tube in the well-known 'flying-spot manner.

Nor is it essential for the code member to be of an optical nature: the code pattem could'altematively be 4 of an inductive nature-such as a magneticpattern along a tape. This particularly applies to: such parallel readout systems, as that of FIG. .6, where the patterns 1 1 and 74 could be injmagnetic form along tape and the photocells replaced by magnetic pickoffs of the flux gate type, suitable for reading stationary tracks.

Or the pattern may be of a capacitive or even a mechanical kind.

What we claim is:

1. Measuring apparatus responsive to the position of a first object movable relative to a secondobject including a code member secured to the second object and carrying a sequence of binary digital elements which extends in'the direction ,of relative movement of said second object with respect to said first object and forming a chain code and is such that any group of a predetermined number N of consecutive elements differs in pattern from any other group of N consecutive elements,

and identifyingmeans located to'respond to said elements and operable at any relative position of the objects within the resolution of the patterns to derive a measurement signal which identifies that position in terms of a particular one of said groups said identifying means including a register, v. T

. 14 v transfer means located'to respond to said elements by transferringito the register the digital pattern of the particular group of. elements which at any 7 given moment represents the said relativeposition of the first object at that moment, 'a code generator operable'on' pulse energization to generate successive ones-of said group patterns, a pulsesource connected to pulse-energize the code generator, i comparison means'interconnected with the register andthe code generator so as to respond when their jpattems are the same, and output means'responsive to said source and the comparison means to derive the said measurement signal from thenumber-of pulses applied to energize the code generator to cause-the comparison means to respond when their patterns are the I same.

2. Apparatus as claimedin claim 1 wherein the output means includes a counter and connecting means for applying to said counter pulses from said source until the comparisonmeans responds as aforesaid, whereby the number :held in the counter at any'given moment represents the said relative position at that moment.

3. Apparatus as claimed in claim 1 where the code member is an optical grating ruled to represent said patterns andwherein the transfer means includes scanning means movable with the second object and operable at any relative .po'sitionof the objects within the. resolution of the patterns to irradiate digit-by digit in turn a'portion of the code member including the particular one of said 7 represents that position,

a photoelectric device disposed to be sequentially irradiated by. said portion as scanned and represent it by a series train of at least said number. of digital signals,

.and group-selecting arrangements for applying to the register the N'digital'signals representing said par- 'ticular group. t

4. Apparatus as claimed in claim .4 including a a further optical grating to enablesaid scanning means to derive a. pulse forstrobing the digital signals derived from the code'member so as to pass to the, register only an approximately central part of each such digital signal.

5. Apparatus as claimed in claim 3 wherein to supplement the said measurement signal there is provided control and reference optical gratings secured to the first and to the second object respectively, further scanning means movable with the second object and operable in synchronism with the firstmentioned scanning means to irradiate those gratings so as to set up similar cyclic control and reference patterns of wavelength proportional to the pitch of the code elements and of relative phase dependent on the relative jects, a I

controland reference photoelectric devices disposed so as to be irradiated by light from those patterns,

means responsive to the relative phase of the control and reference signals from those devices to indicate the relative position of the objects within the space between adjacent ones of said digital elements,

position of the obgroups which and means for causing the control signal to strobe the digital signalsderived from the code member so as to pass to the register-stage only an approximately central part of each such'digital signal. 1

6. Apparatus as claimed in claim 1 wherein the transfer means includes 3 a set of N pickoff devices disposed to respondto the N digital elements of the particular pattern which represents the said relative position of the first object, f

and connection means for applying the outputs from said N pickoff devices to the register. v 7. Apparatus as claimed in claim 6 which includes a further set of N pickoff devices disposed to respond to said elements but displaced by half a pitch of said elements from the set first mentioned, I

a switching stage for selectively connecting said sets to the register,

and means for controlling the switching stage so as to connect to the register at any given moment the particular one of said sets that is the more centrally aligned on said elements at that moment.

8. Apparatus-as claimed in claim 6 where the code member is an optical grating, wherein the pickoff devices are photoelectric devices and means are provided for irradiating the code member to enable the devices to respond to said elements.

9. Apparatus as claimed in claim 7 wherein to supplement the said measurement signal there is provided means for providing control and reference cyclic signals of wavelength proportional to the pitch of the code elements and of relative phase dependent on the relative position of the objects, a

means responsive to the relative phase of said cyclic signals to indicate the relative position of the objects within the space between adjacent ones of said elements,

and means for deriving from those signals at least one signal to control the timing of the said identifying means.

10. Apparatus as claimed in claim 1 wherein the register and the code generator form a combined unit, and further including I a logic network interconnected with the unit to condition it to function either as a shift register or as a code generator. means for actuating the logic network after the f6 response of said transfer means to convert the unit from a register to a code generator whilst retaining the registered code pattern,

.- said comparison means being interconnected with the unit to compare the condition of the unit with a datum condition thereof and respond as aforesaid on receipt by the unit of sufficient of said pulses to restore the generator to that datum condition,

. and means for actuating the logic network to restore the unit to function as a register before the next response of the transfer means. I I

11. Measuring apparatus responsive to the position of first object movable relative to a second object including 1 a code member secured to the second object and carrying a sequence of binary digital elements which extends'in the direction of relative movement of unit to condition it to function either as a shift register or a code generator, transfer means located to respond to said elements by transferring to the unit when conditioned as a shift register the digital pattern of the particular group of N elements which at any given moment represents the said relative position of the first object at that moment, means for actuating the logic network after the response of said transfer means to convert the unit from its register condition to its code generator condition whilst retaining the registered code pattern, a source connected to supply pulses to step the unit when in its code generator condition so as to restore the patterns to a datum pattern, output means responsive to said source to derive a measurement signal which represents the relative position of the first object by the number of pulses required to step the code generator from the registered pattern to the datum pattem, and means for actuating the logic network to restore the unit to its register condition before the next response of the transfer means. 

1. Measuring apparatus responsive to the position of a first object movable relative to a second object including a code member secured to the second objeCt and carrying a sequence of binary digital elements which extends in the direction of relative movement of said second object with respect to said first object and forming a chain code and is such that any group of a predetermined number N of consecutive elements differs in pattern from any other group of N consecutive elements, and identifying means located to respond to said elements and operable at any relative position of the objects within the resolution of the patterns to derive a measurement signal which identifies that position in terms of a particular one of said groups said identifying means including a register, transfer means located to respond to said elements by transferring to the register the digital pattern of the particular group of elements which at any given moment represents the said relative position of the first object at that moment, a code generator operable on pulse energization to generate successive ones of said group patterns, a pulse source connected to pulse-energize the code generator, comparison means interconnected with the register and the code generator so as to respond when their patterns are the same, and output means responsive to said source and the comparison means to derive the said measurement signal from the number of pulses applied to energize the code generator to cause the comparison means to respond when their patterns are the same.
 2. Apparatus as claimed in claim 1 wherein the output means includes a counter and connecting means for applying to said counter pulses from said source until the comparison means responds as aforesaid, whereby the number held in the counter at any given moment represents the said relative position at that moment.
 3. Apparatus as claimed in claim 1 where the code member is an optical grating ruled to represent said patterns and wherein the transfer means includes scanning means movable with the second object and operable at any relative position of the objects within the resolution of the patterns to irradiate digit-by-digit in turn a portion of the code member including the particular one of said groups which represents that position, a photoelectric device disposed to be sequentially irradiated by said portion as scanned and represent it by a series train of at least said number of digital signals, and group-selecting arrangements for applying to the register the N digital signals representing said particular group.
 4. Apparatus as claimed in claim 4 including a further optical grating to enable said scanning means to derive a pulse for strobing the digital signals derived from the code member so as to pass to the register only an approximately central part of each such digital signal.
 5. Apparatus as claimed in claim 3 wherein to supplement the said measurement signal there is provided control and reference optical gratings secured to the first and to the second object respectively, further scanning means movable with the second object and operable in synchronism with the first-mentioned scanning means to irradiate those gratings so as to set up similar cyclic control and reference patterns of wavelength proportional to the pitch of the code elements and of relative phase dependent on the relative position of the objects, control and reference photoelectric devices disposed so as to be irradiated by light from those patterns, means responsive to the relative phase of the control and reference signals from those devices to indicate the relative position of the objects within the space between adjacent ones of said digital elements, and means for causing the control signal to strobe the digital signals derived from the code member so as to pass to the register stage only an approximately central part of each such digital signal.
 6. Apparatus as claimed in claim 1 wherein the transfer means includes a set of N pickoff devices disposed to respond to the N digital elements of the particular patteRn which represents the said relative position of the first object, and connection means for applying the outputs from said N pickoff devices to the register.
 7. Apparatus as claimed in claim 6 which includes a further set of N pickoff devices disposed to respond to said elements but displaced by half a pitch of said elements from the set first mentioned, a switching stage for selectively connecting said sets to the register, and means for controlling the switching stage so as to connect to the register at any given moment the particular one of said sets that is the more centrally aligned on said elements at that moment.
 8. Apparatus as claimed in claim 6 where the code member is an optical grating, wherein the pickoff devices are photoelectric devices and means are provided for irradiating the code member to enable the devices to respond to said elements.
 9. Apparatus as claimed in claim 7 wherein to supplement the said measurement signal there is provided means for providing control and reference cyclic signals of wavelength proportional to the pitch of the code elements and of relative phase dependent on the relative position of the objects, means responsive to the relative phase of said cyclic signals to indicate the relative position of the objects within the space between adjacent ones of said elements, and means for deriving from those signals at least one signal to control the timing of the said identifying means.
 10. Apparatus as claimed in claim 1 wherein the register and the code generator form a combined unit, and further including a logic network interconnected with the unit to condition it to function either as a shift register or as a code generator. means for actuating the logic network after the response of said transfer means to convert the unit from a register to a code generator whilst retaining the registered code pattern, said comparison means being interconnected with the unit to compare the condition of the unit with a datum condition thereof and respond as aforesaid on receipt by the unit of sufficient of said pulses to restore the generator to that datum condition, and means for actuating the logic network to restore the unit to function as a register before the next response of the transfer means.
 11. Measuring apparatus responsive to the position of first object movable relative to a second object including a code member secured to the second object and carrying a sequence of binary digital elements which extends in the direction of relative movement of said second object with respect to said first object forming a chain code and is such that any group of a predetermined number N of consecutive elements differs in pattern from any other group of N consecutive elements, and identifying means including a combined shift register and code generator unit, a logic network interconnected with the unit to condition it to function either as a shift register or a code generator, transfer means located to respond to said elements by transferring to the unit when conditioned as a shift register the digital pattern of the particular group of N elements which at any given moment represents the said relative position of the first object at that moment, means for actuating the logic network after the response of said transfer means to convert the unit from its register condition to its code generator condition whilst retaining the registered code pattern, a source connected to supply pulses to step the unit when in its code generator condition so as to restore the patterns to a datum pattern, output means responsive to said source to derive a measurement signal which represents the relative position of the first object by the number of pulses required to step the code generator from the registered pattern to the datum pattern, and means for actuating the logic network to restore the unit to its register condition before the next response of the transfer means. 